1. Field of the Invention
The present invention relates generally to a canister which collects fuel vapor generated in a fuel tank of a vehicle, thereby preventing the fuel vapor from leaking to the atmosphere.
2. Description of the Related Art
Conventionally, canisters have been used to prevent a fuel component contained in fuel vapor generated in vehicle fuel tanks from leaking to the atmosphere.
For example, Japanese Unexamined Patent Publication No. Hei 5-33734 discloses a technique to treat fuel vapor by using a canister more efficiently. In this technique, the interior of a canister is divided by a vertically extended partition into two chambers having different volumes. Both chambers accommodate an adsorbing material to adsorb the fuel component of fuel vapor. The chamber having a larger volume serves as a main chamber, while the chamber having a smaller volume serves as a sub chamber. The main chamber communicates with a fuel tank via a fuel vapor passage and also with the intake system of an engine via a purge pipe. The sub chamber is provided with an air introducing port for introducing air into the canister when purge control is performed. The main and sub chambers communicate with each other through a communication passage formed under the chambers. The partition forms a throttle in the communication passage at the boundary between the main chamber and the sub chamber to decrease the cross section of the communication passage.
Fuel vapor generated in the fuel tank is first led to the main chamber of the canister via the fuel vapor passage, so that the fuel component of the fuel vapor is collected by the adsorbing material in the main chamber. After passing through the main chamber, the fuel vapor is led to the sub chamber via the communication passage provided under the main chamber. The remaining fuel component of the fuel vapor which has not been collected by the adsorbing material in the main chamber is collected by the adsorbing material in the sub chamber.
When the fuel vapor flows from the main chamber to the sub chamber through the communication passage, the fuel vapor flows along a generally U-shaped path. This prolongs the period of time during which the fuel vapor contacts the adsorbing material, thereby increasing efficiency in collecting the fuel component. When the fuel vapor flows from the main chamber to the sub chamber, the fuel vapor encounters passage resistance due to the throttle of the communication passage, so that the flow of the fuel vapor is restricted.
Accordingly, the amount of the fuel component that is to be adsorbed by the adsorbing material in the sub chamber decreases, and the adsorbing material in the sub chamber an therefore adsorb the fuel component while reserving some adsorbing capacity. As a result, the fuel vapor introduced to the sub chamber is released to the atmosphere after the fuel component has been sufficiently adsorbed by the adsorbing material. As described above, in the prior art technique, the ability of the canister to treat fuel vapor is increased by providing a throttle in the communication passage through which the fuel vapor passes.
Recently, fuel vapor leaking from fuel tanks through the fuel filling opening to the atmosphere during the filling process is considered to be a significant cause of air pollution.
U.S. Pat. No. 4,714,172 discloses a technique to solve the above-described problem. In this technique, a canister and a fuel tank are connected with each other through a breather passage. Also, a differential pressure valve is disposed in the middle of the breather passage. The differential pressure valve opens when the tank is being filled with fuel. A seal is further provided inside a fuel filling tube. Therefore, when a fill nozzle is inserted into the fuel filling tube, the periphery of the fill nozzle is sealed with respect to the tube.
When the tank is filled with fuel, the internal pressure of the fuel tank increases so that the differential pressure valve opens. Accordingly, fuel vapor flows in the breather passage from the fuel tank to the canister. The fuel component of the fuel vapor is collected by the adsorbing material in the canister. At this time, the periphery of the fill nozzle is sealed by the seal of the fuel filling tube, so that the fuel vapor in the fuel tank is prevented from leaking outside through the fuel filling opening. When this technique is used, it is possible to fill the fuel tank with fuel without allowing fuel vapor to leak from the fuel tank to the outside.
The process of leading fuel vapor, which is generated in a fuel tank upon filling, to a canister in order to collect the fuel component of the fuel vapor is hereinafter referred to as an ORVR (Onboard Refueling Vapor Recovery) treatment.
The amount of fuel vapor that flows into the canister during ORVR treatment (about 45 liters/min) is relatively large compared with the amount of fuel vapor which flows into the canister when the tank is not being filled with fuel (about 1 liter/min). Also, the flow rate of fuel vapor within the canister increases during ORVR treatment. Accordingly, if ORVR treatment is performed using a conventional canister, the following problems occur:
(1) Since the flow resistance of the passage is large in conventional canisters, it is impossible to introduce a large amount of fuel vapor from the fuel tank into the canister during ORVR treatment. Accordingly, the flow of fuel vapor from the fuel tank to the canister is hindered, and the internal pressure of the fuel tank increases. As a result, there is the possibility that filling the fuel tank will be difficult.
(2) Since the flow rate of fuel vapor is large, the fuel vapor may flow unevenly when the fuel component of the fuel vapor is recovered by the canister, so that only part of the fuel vapor will contact the adsorbing material. Accordingly, even when the entire canister has enough adsorbing capacity in reserve, the part of the adsorbing material that the fuel vapor contacts in a concentrated manner reaches a limit in adsorbing performance. As a result, it is possible that fuel vapor from which the fuel component has not been sufficiently recovered is released to the outside.